La Niņa From A Canadian Perspective
Ray GarnettCanadian Wheat Board
Winnipeg, Mantoba, Canada
rgarnett@cgc.ca
Introduction
The pervasive impact of El Niņo/Southern Oscillation (ENSO) on world weather patterns in general and on the North American weather and climate has been a subject of intense study in the last 15 years or so (Ropelewski and Halpert, 1987, 1989; Kiladis and Diaz, 1989). These and many other studies reported in recent literature have helped to identify temperature and precipitation anomalies associated with El Niņo (the warm phase of ENSO) in various parts of the world in general and over the conterminous USA in particular. The impacts of El Niņo and its counterpart La Niņa (the cold phase of ENSO) on Canadian temperature and precipitation patterns have been documented in two recent studies (Shabbar and Khandekar, 1995; Shabbar, Bosnal and Khandekar, 1997). In general it can be stated that El Niņo brings warmer and drier weather over western Canada and La Niña brings colder and wetter weather to this region, specifically during the winter following the onset of El Niņo or of La Niņa, respectively.The impact of El Niņo and La Niņa on Canadian agriculture and, in particular, on the Canadian wheat yield has been studied by Garnett and Khandekar (1992) among others. In general, it has been found that El Niņo years tend to favor the wheat yield, while La Niņa years tend to be unfavorable for the wheat yield. Specifically, when the sea surface temperatures (SSTs) in the central and eastern equatorial Pacific are above normal, during winter and early spring, the rainfall over the Canadian Prairies during the months of June and July (the wheat growing season) is above normal which in turn favors the wheat yield.
The SST distribution in the central and eastern equatorial Pacific influences the central and north Pacific atmospheric flow patterns, as documented in a landmark paper by Wallace and Gutzler (1981) who have identified the PNA (Pacific North American) flow patterns for which suitable PNA indices have been developed. In a recent study (Garnett, Khandekar and Babb, 1998) the utility of ENSO and PNA indices for long-lead forecasting of summer weather over the Canadian Prairies has been demonstrated.
El Niņo, La Niņa and Canadian Wheat Yields
As mentioned earlier, El Niņo years are found to be favorable and La Niña years unfavorable for the wheat yield over the Canadian Prairies. As an example, the wheat yield series for the Canadian Prairies, as shown in Table 1, has the El Niņo (La Niņa) years identified as the basis for a number of classification schemes.
Impact of Other Large-Scale Features on Canadian Grain Yields
Garnett and Khandekar (1992) also analyzed the impact of large-scale features like the Equatorial Stratospheric Quasi Biennial Wind Oscillation (or QBO as it is commonly known), Eurasian winter snow and Indian summer (June-September) monsoon rainfall. Interestingly, the westerly (easterly) phase of the QBO is found to influence the Indian summer monsoon rainfall favorably, while the Indian summer monsoon rainfall is found to be strongly and inversely correlated with USA corn yield, but only weakly (and inversely) correlated with Canadian wheat yield. The influence of ENSO, QBO and Eurasian winter snow cover on the Indian Monsoon has been studied by Khandekar (1996) and Khandekar and Nerella (1984) among others. This and many other studies have shown that, in general, El Niņo events tend to be unfavorable and La Niña favorable for the Indian Monsoon during the mid-summer season in a given year. Thus, it may be possible to foreshadow the USA corn yield and the Canadian wheat yield with a lead time of 1 to 3 months. Serious droughts like the ones in 1983 and 1988 in the USA corn belt occurred simultaneously with a flood monsoon in India. The severe drought of 1961 on the Canadian Prairies also occurred during a severe flood monsoon in India. Hence, there is value in closely monitoring the various forcing functions that govern the performance of the Indian Monsoon.
Concluding Remarks
El Niņo and La Niņa appear to provide a strong climatic signal on the climate and, therefore, agriculture in western Canada. Recent studies have documented this impact through empirical and correlation analyses. Continuation of such studies will help improve our capability for long-lead forecasting of grain yields over the Canadian Prairies with a lead time of 3 to 6 months.
References
Garnett, E.R. and M.L. Khandekar, 1992: The impact of large-scale atmospheric circulation and anomalies on Indian monsoon droughts and floods and on world grain yields - statistical analysis. J. Agriculture and Forest Meteorology, 61, 113-128.Garnett, E.R., M.L. Khandekar and J.C. Babb, 1998: On the Utility of ENSO and PNA Indices for the Long Lead Forecasting of Summer Weather over the Crop Growing Region of the Canadian Prairies. Journal of Theoretical and Applied Climatology, 60, 37-45.
Khandekar, M.L. and V.R. Nerella, 1984: On The Relationship Between The Sea Surface Temperatures In The Equatorial Pacific and The Indian Monsoon Rainfall. Geophysical Research Letters, 11, 1137-1140.
Khandekar, M.L., 1996: El Niņo/Southern Oscillation, Indian Monsoon and World Grain Yields-A Synthesis. Kluwer Academic Publishers (Printed in the Netherlands). M.I. El Sabh et al (eds.) Land-based and Marine Hazards, 79-95. (Advances in Natural and Technical Hazards Research, 7).
Kiladis, G.N. and H.F. Diaz, 1989: Global climate anomalies associated with extremes in the Southern Oscillation. Journal of Climate, 2, 1069-1090.
Ropelewski, C.P. and M.S. Halpert, 1987: Global and regional scale precipitation patterns associated with El Niņo/Southern Oscillation. Monthly Weather Review, 115, 1606-1626.
Ropelewski, C.P. and M.S. Halpert, 1989: Precipitation patterns associated with the high index phase of the Southern Oscillation. Journal of Climate, 2, 268-284.
Shabbar, A. and M.L. Khandekar, 1995: The Impact of El Niņo-Southern Oscillation on the Temperature Field over Canada. Atmosphere-Ocean 34, 2, 1996. 461-416
Shabbar, A., B. Bonsal and M.L. Khandekar, 1997: Canadian Precipitation Patterns Associated with the Southern Oscillation Journal of Climate. Journal of Climate, 10, 3016-3027.
Wallace, J.M. and D.S. Gutzler, 1981: Teleconnections in the geopotential height field during the northern hemisphere winter. Monthly Weather Review, 109, 784-812.
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